1. Field of the Invention
The present invention relates to a method of making a green compact of a rare earth alloy magnetic powder and a method of producing a rare earth permanent magnet.
2. Description of the Related Art
A rare earth alloy sintered magnet is produced by pulverizing a rare earth alloy into a magnetic alloy powder, pressing and compacting the powder into a green compact in a desired shape and then subjecting green compact to sintering and aging processes. Currently, rare earth alloy sintered magnets have found a broad variety of applications and are typically made of either a samarium-cobalt compound or a neodymium-iron-boron compound. A neodymium-iron-boron magnet (which will be herein called an xe2x80x9cRxe2x80x94Txe2x80x94B magnetxe2x80x9d), in particular, has a higher maximum energy product than a magnet of any other type, and yet is available at a reasonable price. Accordingly, Rxe2x80x94Txe2x80x94B magnets have been used for various kinds of electronic appliances with increasing frequency. In an Rxe2x80x94Txe2x80x94B magnet, R is a rare earth element including Y, T is either iron or a compound of iron and a transition metal (e.g., Co) in which iron is partially replaced with the metal, and B is boron. Part of boron can be replaced with carbon.
To prepare such a rare earth alloy, an ingot casting process has been used. In an ingot casting process, a molten material alloy is poured (or teemed) into ingot casting molds and then cooled down relatively slowly. The alloy ingot, once formed by this ingot casting process, is pulverized into an alloy powder by a known technique. Next, the resultant alloy powder is pressed and compacted by various types of powder presses, forming a green compact. Finally, the green compact is loaded into a furnace chamber for sintering.
Recently, however, a rapid quenching process, like strip casting or centrifugal casting, has been preferred. In a rapid quenching process, a solidified alloy strip or flake, thinner than an alloy ingot, can be made from a molten alloy by contacting the melt with single or twin roller, rotating disk or rotating cylindrical mold, for example, so that the alloy is quenched relatively rapidly. An alloy strip prepared by a process like this generally has a thickness of 0.03 mm to 10 mm. According to the rapid quenching process, the molten alloy starts to be solidified at the surface being in contact with the chill roller (which will be herein called a xe2x80x9croller-alloy contact surfacexe2x80x9d). Then, columnar crystals grow from the roller-alloy contact surface in the thickness direction, or outward. Accordingly, when prepared by a strip casting method, for example, a rapidly solidified alloy has a structure including a combination of R2T14B crystal phases and R-rich phases. Normally, the sizes of each of the R2T14B crystal phases are from 0.1 xcexcm through 100 xcexcm in the minor axis direction and from 5 xcexcm through 500 xcexcm in the major axis direction. The R-rich phases exist dispersively around the grain boundaries of the R2T14B crystal phases. Also, each of the R-rich phases is a non-magnetic phase in which the concentration of the rare earth element R is relatively high, and has a thickness of 10 xcexcm or less, corresponding to the width of the associated grain boundary.
In a rapid quenching process, an alloy is quenched and solidified in a shorter time (at a cooling rate between 102xc2x0 C./sec. and 104xc2x0 C./sec.) compared to the conventional ingot casting process. Thus, the rapidly solidified alloy can have a finer micro-structure and a smaller crystal grain size. In addition, the grain boundary (or intergranular phases) of the alloy of this type has a broader area and includes a thin layer of R-rich phases. As a result, the rapidly solidified alloy advantageously exhibits a wider dispersion of R-rich phases.
However, the present inventors found that if a magnetic powder of a rapidly solidified alloy (e.g., a strip cast alloy, typically) is compacted by a known pressing technique, the as-pressed, green compact has a potential to generate sufficient heat for combustion, depending on the particular state of the environment. This is probably because easily oxidizable R-rich phases are often exposed on the surface of powder particles of the rapidly solidified alloy, thus making the powder of the rapidly solidified alloy subject to oxidation and the resultant heat therefrom. Also, even if the heat from the oxidation of the powder is insufficient to cause combustion, the oxidization may deteriorate the magnetic properties of resultant magnets.
The heat generation resulting from the oxidization of rare earth elements is also observable when the powder of a rare earth alloy, prepared by a known ingot casting process, is pressed and compacted. However, the heat generation is markedly increased when the pressed and compacted powder is made from a rapidly solidified alloy (e.g., a strip cast alloy, in particular). Accordingly, even though a rapidly solidified alloy powder has a finer structure and potentially contributes to better magnetic properties, the rapid quenching process is still unqualified for mass production so long as there is any risk of heat generation or combustion left during the pressing.
It is possible to suppress oxidation of the rare earth alloy powder by carrying out the pressing and compacting process within an inert gas environment. However, pressing within an inert gas environment is far from a practical approach to the oxidation problem. This is because even though a pressing process can be performed fully automatically using a compacting machine, the process itself still requires frequent maintenance. That is to say, workers often have to check the presses. For example, in the event that a press placed within an inert gas (e.g., N2) environment fails, a worker must tend to the machine. However, the worker must either bring his own supply of oxygen, or he must replace the inert gas environment with a breathable environment. Moreover, placing the press entirely within such an inert gas environment requires an large amount of inert gas. Accordingly, this approach is neither cost-effective nor practical.
It is therefore an object of the present invention to provide a method of making a green compact of a rare earth alloy magnetic powder in such a manner as to avoid the combustion accidents and to attain superior magnetic properties even when the powder is easily oxidizable.
It is another object of the invention to provide a method of producing a rare earth permanent magnet by utilizing the inventive powder compacting method.
According to an embodiment of the powder compacting method of the present invention, a green compact of a rare earth alloy magnetic powder is made by pressing the powder within an air environment that has a temperature controlled at 30xc2x0 C. or less and a relative humidity controlled at 65% or less.
According to another embodiment of the compacting method of the present invention, a green compact of a rare earth alloy magnetic powder is pressed in an air environment that also has a temperature controlled at 30xc2x0 C. or less. The temperature minus a dew point is controlled at 6xc2x0 C. or more. As used herein, the xe2x80x9cdew pointxe2x80x9d is the temperature at which a given parcel of air is saturated with water vapor.
In one embodiment of the compacting method of the present invention, the powder may be prepared by pulverizing a rapidly solidified alloy that has been obtained by quenching a molten alloy at a rate from 102xc2x0 C./sec. through 104xc2x0 C./sec.
In this particular embodiment, the rapidly solidified alloy is a rare earth alloy with a thickness between 0.03 mm and 10 mm, and preferably includes R2T14B crystal grains (where R is a rare earth element, T is either iron or a compound of iron and a transition metal element in which iron is partially replaced with the metal, and B is boron) and R-rich phases. The sizes of the R2T14B crystal grains are preferably from 0.1 xcexcm to 100 xcexcm in a minor axis direction, and from 5 xcexcm to 500 xcexcm a major axis direction. The R-rich phases are dispersed around a boundary of the R2T14B crystal grains.
In another embodiment of the present invention, a lubricant is preferably added to the powder being pressed.
In still another embodiment of the present invention, oxygen contained in the powder is preferably limited to 6,000 ppm or less by weight.
In yet another embodiment of the present invention, the rapidly solidified alloy is finely pulverized using a jet mill with the concentration of an oxidizing gas controlled in a pulverization chamber, thereby forming an oxide layer on the surface of particles of the finely pulverized powder.
In yet another embodiment of the present invention, the alloy powder is pressed in an air environment that also has a temperature controlled at 5xc2x0 C. or more and has a relative humidity controlled at 40% or more. The alloy powder is pressed in an air environment that also has a temperature controlled at 30xc2x0 C. or less
More preferably, the alloy powder is pressed in an air environment that has a temperature controlled at a point between 15xc2x0 C. and 25xc2x0 C., and a relative humidity controlled at a point between 40% and 55%.
In a preferred embodiment of the present invention, a die pressing machine is used. The machine includes: a die with a die hole for forming at least part of a cavity therein; and first and second punches for compacting the powder inside the hole. The method preferably includes the step of filling the cavity with the powder with at least an upper end of the second punch inserted into the die hole. The method further includes the steps of: inserting at least a lower end of the first punch into the die hole and compacting the powder between the first and second punches, thereby making the green compact of the powder; and ejecting the compact out of the die hole.
An embodiment of the present invention for producing a rare earth permanent magnet includes the steps of: preparing the green compact of the rare earth alloy magnetic powder according to any embodiment of the inventive powder compacting method; and sintering the compact.
In one embodiment of the present invention, after the powder has been pressed to make the green compact in a first chamber having the air environment, the compact is transported to a second chamber having an environment at a controlled temperature, which is different from the temperature of the air environment by 5xc2x0 C. or less, and then sintered in the second chamber. In this particular embodiment, the first chamber is preferably big enough for a human being to work therein.